Network Working Group                                      M. Jones, Ed.
Internet-Draft                                                 Microsoft
Intended status: Standards Track                         E. Hammer-Lahav                                D. Hardt
Expires: June 3, 4, 2011                                             Yahoo!
                                                                D. Hardt                                        independent
                                                             D. Recordon
                                                                Facebook
                                                       November 30,
                                                        December 1, 2010

                 The OAuth 2.0 Protocol: Bearer Tokens
                     draft-ietf-oauth-v2-bearer-00
                     draft-ietf-oauth-v2-bearer-01

Abstract

   This specification describes how to use bearer tokens when accessing
   OAuth 2.0 protected resources.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 3, 4, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Notational Conventions . . . . . . . . . . . . . . . . . .  3
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
     1.3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Authenticated Requests . . . . . . . . . . . . . . . . . . . .  4
     2.1.  The Authorization Request Header Field . . . . . . . . . .  5
     2.2.  URI Query  Form-Encoded Body Parameter  . . . . . . . . . . . . . . .  5
     2.3.  URI Query Parameter  . . . .  5
     2.3.  Form-Encoded Body Parameter . . . . . . . . . . . . . . .  6
     2.4.  The WWW-Authenticate Response Header Field
   3.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
     2.5.  Error Codes
     3.1.  Security Threats . . . . . . . . . . . . . . . . . . . . .  7
     3.2.  Threat Mitigation  . . . . .  8
   3.  Security Considerations . . . . . . . . . . . . . . .  7
     3.3.  Summary of Recommendations . . . .  9
     3.1.  Validate SSL Certificate Chains . . . . . . . . . . . .  8
   4.  IANA Considerations  . . .  9
     3.2.  Always use TLS (https) . . . . . . . . . . . . . . . . . .  9
     3.3.  Don't store bearer tokens in cookies
   5.  References . . . . . . . . . . . . .  9
     3.4.  Issue short-lived bearer tokens . . . . . . . . . . . . .  9
   Appendix A.  Acknowledgements
     5.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
   Appendix B.  Document History
     5.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   4.  Normative References
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 10
   Appendix B.  Document History  . . . 10 . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11

1.  Introduction

   OAuth enables clients to access protected resources by obtaining an
   access token (a token, which is defined in [OAuth2] as "a string that denotes a specific scope, duration, and
   other attributes), representing
   an access authorization issued to the client", rather than using the
   resource owner's credentials.

   Tokens are issued to third-party clients by an authorization server with the
   approval of the resource owner.  The client uses the access token to
   access the protected resources hosted by the resource server.  This
   specification describes how to make protected resource requests by
   treating an OAuth access token as a bearer token.

   This specification defines the use of bearer tokens with OAuth over
   HTTP [RFC2616] (or
   HTTP over using TLS as defined by [RFC2818]). [RFC2818].  Other specifications may extend
   it for use with other transport protocols.

1.1.  Notational Conventions

   The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT',
   'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY', and 'OPTIONAL' in this
   document are to be interpreted as described in [RFC2119].

   This document uses the Augmented Backus-Naur Form (ABNF) notation of
   [I-D.ietf-httpbis-p1-messaging].  Additionally, the following rules
   are included from [RFC2617]: realm, auth-param; from [RFC3986]: URI-
   Reference; and from
   [I-D.ietf-httpbis-p1-messaging]: OWS, RWS, and
   quoted-string. RWS.

   Unless otherwise noted, all the protocol parameter names and values
   are case sensitive.

1.2.  Terminology

   All terms are as defined in The OAuth 2.0 Protocol [OAuth2].

1.3.  Overview

   OAuth provides a method for clients to access a protected resource on
   behalf of a resource owner.  Before a client can access a protected
   resource, it must first obtain authorization (access grant) from the
   resource owner, then exchange the access grant for an access token
   (representing the grant's scope, duration, and other attributes).
   The client accesses the protected resource by presenting the access
   token to the resource server.

   The access token provides an abstraction layer, replacing different
   authorization constructs (e.g. username and password, assertion) for
   a single token understood by the resource server.  This abstraction
   enables issuing access tokens valid for a short time period, as well
   as removing the resource server's need to understand a wide range of
   authentication schemes.

   +--------+                               +---------------+
   |        |--(A)- Authorization Request ->|   Resource    |
   |        |                               |     Owner     |
   |        |<-(B)----- Access Grant -------|               |
   |        |                               +---------------+
   |        |
   |        |           Access Grant &      +---------------+
   |        |--(C)--- Client Credentials -->| Authorization |
   | Client |                               |     Server    |
   |        |<-(D)----- Access Token -------|               |
   |        |                               +---------------+
   |        |
   |        |                               +---------------+
   |        |--(E)----- Access Token ------>|    Resource   |
   |        |                               |     Server    |
   |        |<-(F)--- Protected Resource ---|               |
   +--------+                               +---------------+

                     Figure 1: Abstract Protocol Flow

   The abstract flow illustrated in Figure 1 describes the overall OAuth
   2.0 protocol architecture.  The following steps are specified within
   this document:

      E) The client makes a protected resource request to the resource
      server by presenting the access token.

      F) The resource server validates the access token, and if valid,
      serves the request.

2.  Authenticated Requests

   Clients make authenticated token requests using the "Authorization"
   request header field.  Resource servers MUST accept authenticated
   requests using the "OAuth" "OAuth2" HTTP authentication scheme as described
   in Section 2.1, and MAY support additional methods.

   Alternatively, clients MAY attempt to include the access token using in the
   HTTP request URI in body when using the query component "application/x-www-form-urlencoded" content
   type as described in Section 2.2, 2.2 or in using the HTTP body when using request URI in the
   "application/x-www-form-urlencoded" content type
   query component as described in Section 2.3.  Resource server servers MAY
   support these alternative methods.

   Clients SHOULD only use the request URI or body or URI when the
   "Authorization" request header field is not available, and MUST NOT
   use more than one method to transport the token in each request.
   Because of the Security Considerations (Section 3) associated with
   the URI method, it SHOULD only be used if no other method is
   feasible.

2.1.  The Authorization Request Header Field

   The "Authorization" request header field is used by clients to make
   authenticated token requests.  The client uses the "OAuth" "OAuth2"
   authentication scheme to include the access token in the request.

   For example:

   GET /resource HTTP/1.1
   Host: server.example.com
   Authorization: OAuth OAuth2 vF9dft4qmT

   The "Authorization" header field uses the framework defined by
   [RFC2617] as follows:

   credentials    = "OAuth" "OAuth2" RWS access-token [ RWS 1#auth-param ]
   access-token   = 1*( quoted-char / <"> )

   quoted-char    =   "!" / "#" / "$" / "%" / "&" / "'" / "("
                    / ")" / "*" / "+" / "-" / "." / "/" / DIGIT
                    / ":" / "<" / "=" / ">" / "?" / "@" / ALPHA
                    / "[" / "]" / "^" / "_" / "`" / "{" / "|"
                    / "}" / "~" / "\" / "," / ";"

      NOTE: [RFC5849] defines a different format for the "OAuth"
      authentication scheme.  Resource servers can differentiate between
      the two protocol versions based on the presence of the
      "oauth_signature_method" which is REQUIRED in the previous version
      and is not supported by this specification.

2.2.  URI Query  Form-Encoded Body Parameter

   When including the access token in the HTTP request URI, entity-body, the
   client adds the access token to the request URI query component as defined
   by [RFC3986] body using the
   "oauth_token" parameter.

   For example, the client makes the following HTTP request using
   transport-layer security:

   GET /resource?oauth_token=vF9dft4qmT HTTP/1.1
   Host: server.example.com

   The HTTP request URI query can include other request-specific
   parameters, in which case, the "oauth_token" parameters SHOULD be
   appended following the request-specific parameters, properly
   separated by an "&" character (ASCII code 38).

   For example:

   http://example.com/resource?x=y&oauth_token=vF9dft4qmT

      NOTE: The "oauth_token" parameter is used by the previous version
      of the OAuth protocol as described in [RFC5849].  Resource servers
      can differentiate between the two protocol versions based on the
      presence of the "oauth_signature_method" which is REQUIRED in the
      previous version and is not supported by this specification.

2.3.  Form-Encoded Body Parameter

   When including the access token in the HTTP request entity-body, the
   client adds the access token to the request body using the
   "oauth_token" parameter.  The  The client can use this method only if the
   following REQUIRED conditions are met:

   o  The HTTP request entity-body is single-part.

   o  The entity-body follows the encoding requirements of the
      "application/x-www-form-urlencoded" content-type as defined by
      [W3C.REC-html401-19991224].

   o  The HTTP request entity-header includes the "Content-Type" header
      field set to "application/x-www-form-urlencoded".

   o  The HTTP request method is "POST", "PUT", or "DELETE". one for which a body is permitted to be
      present in the request.  In particular, this means that the "GET"
      method MAY NOT be used.

   The entity-body can include other request-specific parameters, in
   which case, the "oauth_token" parameters SHOULD be appended following
   the request-specific parameters, properly separated by an "&"
   character (ASCII code 38).

   For example, the client makes the following HTTP request using
   transport-layer security:

   POST /resource HTTP/1.1
   Host: server.example.com
   Content-Type: application/x-www-form-urlencoded

   oauth_token=vF9dft4qmT

      NOTE:

   The "oauth_token" parameter is "application/x-www-form-urlencoded" method should typically only
   be used by in application contexts where participating browsers do not
   have access to the previous version
      of "Authorization" request header field.

2.3.  URI Query Parameter

   When including the OAuth protocol as described access token in [RFC5849].  Resource servers
      can differentiate between the two protocol versions based on HTTP request URI, the
      presence of client
   adds the "oauth_signature_method" which is REQUIRED in access token to the
      previous version and is not supported request URI query component as defined
   by this specification.

2.4.  The WWW-Authenticate Response Header Field

   If [RFC3986] using the protected resource request contains an invalid access token or
   is malformed, "oauth_token" parameter.

   For example, the resource server MUST include client makes the following HTTP
   "WWW-Authenticate" response header field. request using
   transport-layer security:

   GET /resource?oauth_token=vF9dft4qmT HTTP/1.1
   Host: server.example.com

   The "WWW-Authenticate"
   header field uses HTTP request URI query can include other request-specific
   parameters, in which case, the framework defined by [RFC2617] as follows:

   challenge       = "OAuth" RWS token-challenge

   token-challenge = realm
                     [ CS error ]
                     [ CS error-desc ]
                     [ CS error-uri ]
                     [ CS scope ]
                     [ CS 1#auth-param ]

   error           = "error" "=" <"> token <">
   error-desc      = "error_description" "=" quoted-string
   error-uri       = "error_uri" = <"> URI-Reference <">
   scope           = quoted-value /
                     <"> quoted-value *( 1*SP quoted-value ) <">
   quoted-value    = 1*quoted-char

   CS              = OWS "," OWS "oauth_token" parameters SHOULD be
   appended following the request-specific parameters, properly
   separated by an "&" character (ASCII code 38).

   For example:

   HTTP/1.1 401 Unauthorized
   WWW-Authenticate:

   http://example.com/resource?x=y&oauth_token=vF9dft4qmT

   Because of the Security Considerations (Section 3) associated with
   the URI method, it SHOULD only be used if no other method is
   feasible.

3.  Security Considerations

   This section describes the relevant security threats regarding token
   handling when using bearer tokens and describes how to mitigate these
   threats.

3.1.  Security Threats

   The following list presents several common threats against protocols
   utilizing some form of tokens.  This list of threats is based on NIST
   Special Publication 800-63 [NIST800-63].  Since this document builds
   on the OAuth realm="Example Service",
   error="invalid_token",
   error_description="The 2.0 specification, we exclude a discussion of threats
   that are described there or in related documents.

   Token manufacture/modification:  An attacker may generate a bogus
      token or modify the token contents (such as the authentication or
      attribute statements) of an existing token, causing the resource
      server to grant inappropriate access to the client.  For example,
      an attacker may modify the token expired"

   The "realm" to extend the validity period; a
      malicious client may modify the assertion to gain access to
      information that they should not be able to view.

   Token disclosure:  Tokens may contain authentication and attribute is
      statements that include sensitive information.

   Token redirect:  An attacker uses the token generated for consumption
      by resource server to obtain access to another resource server.

   Token reuse:  An attacker attempts to use a token that has already
      been used once with that resource server in the past.

3.2.  Threat Mitigation

   A large range of threats can be mitigated by protecting the contents
   of the token by using a digital signature or a keyed message digest.
   Alternatively, the contents of the token could be passed by reference
   rather than by value (requiring a separate message exchange to
   resolve the reference to the token contents).

   This document does not specify the encoding or the contents of the
   token; hence detailed recommendations for token integrity protection
   are outside the scope of this document.  We assume that the token
   integrity protection is sufficient to prevent the token from being
   modified.

   To deal with token redirect, it is important for the authorization
   server to include the identity of the intended recipients, namely a
   single resource server (or a list of resource servers).  Restricting
   the use of the token to a specific scope is also recommended.

   To provide protection against token disclosure, confidentiality
   protection is applied via TLS with a ciphersuite that offers
   confidentiality protection.  This requires that the protected resources
   partition communication
   interaction between the client and the authorization server, as defined by [RFC2617]. [[ add explanation ]]

   The "error" attribute well
   as the interaction between the client and the resource server,
   utilize confidentiality protection.  Encrypting the token contents is
   another alternative.  Since TLS is used mandatory to provide implement and to use
   with this specification, it is the preferred approach for preventing
   token disclosure via the communication channel.  For those rare cases
   where the client with is prevented from observing the reason
   why contents of the access request was declined.  The parameter values are
   described in Section 2.5.

   The "error_description" attribute provides a human-readable text
   containing additional information, used
   token, token encryption has to assist be applied in addition to the
   understanding and resolution usage of the error occurred.

   The "error_uri" attribute provides a URI identifying a human-readable
   web page
   TLS protection.

   To deal with information about the error, used to offer token reuse, the end-user
   with additional information about following recommendations are made:
   First, the error.  If lifetime of the value is not an
   absolute URI, it is relative token has to be limited by putting a
   validity time field inside the URI of the requested protected
   resource.

   The "scope" attribute is a space-delimited list part of scope values
   indicating the required scope token.  Note
   that using short-lived (one hour or less) tokens significantly
   reduces the impact of one of them being leaked.  Second,
   confidentiality protection of the access token for accessing exchanges between the
   requested resource.

2.5.  Error Codes

   When a request fails, client and
   the resource authorization server responds using the
   appropriate HTTP status code (typically, 400, 401, or 403), and
   includes one of between the following error codes in client and the response:

   invalid_request
         The request is missing resource
   server MUST be applied.  As a required parameter, includes an
         unsupported parameter or parameter value, repeats consequence, no eavesdropper along the same
         parameter, uses more than one method for including an access
         token, or
   communication path is otherwise malformed.  The resource server SHOULD
         respond with able to observe the HTTP 400 (Bad Request) status code.

   invalid_token
         The access token provided is expired, revoked, malformed, or
         invalid for other reasons.  The exchange.
   Consequently, such an on-path adversary cannot replay the token.
   Furthermore, the resource SHOULD respond with server MUST ensure that it only hands out
   tokens to clients it has authenticated first and authorized.  For
   this purpose, the HTTP 401 (Unauthorized) status code.  The client MAY
         request a new access token MUST be authenticated and retry authorized by the protected
   resource
         request.

   insufficient_scope server.  The request requires higher privileges than provided by authorization server MUST also receive a
   confirmation (the consent of the
         access token.  The resource server SHOULD respond with owner) prior to providing a
   token to the HTTP
         403 (Forbidden) status code and MAY include client.  Furthermore, when presenting the "scope"
         attribute with token to a
   resource server, the client MUST verify the identity of that resource
   server.  Note that the scope necessary to access client MUST validate the TLS certificate chain
   when making these requests to protected
         resource.

   [[ Add mechanism for extending error codes ]]

   If the request lacks any authentication information (i.e. resources.  Presenting the client
   was unaware authentication is necessary or attempted using
   token to an
   unsupported authentication method), the unauthenticated and unauthorized resource server SHOULD not
   include an error code or other error information.

   For example:

   HTTP/1.1 401 Unauthorized
   WWW-Authenticate: OAuth realm="Example Service"

3.  Security Considerations

   Implementers
   failing to validate the certificate chain will allow adversaries to
   steal the token and deployers must gain unauthorized access to protected resources.

3.3.  Summary of Recommendations

   Safeguard bearer tokens  Client implementations MUST ensure that
      bearer tokens are not leaked to unintended parties, as they will
      be able to use them to gain access to protected resources.  This
      is the primary security consideration when using bearer tokens
      with OAuth and underlies all the more specific statements recommendations
      that follow.

3.1.

   Validate SSL Certificate Chains certificate chains  The client must validate the TLS
      certificate chain when making requests to protected resources.
      Failing to do so may enable DNS hijacking attacks to steal the
      token and gain unintended access.

3.2.

   Always use TLS (https)  Clients must MUST always use TLS (https) when
      making requests with bearer tokens.  Failing to do so exposes the
      token to numerous attacks that could give attackers unintended
      access.

3.3.

   Don't store bearer tokens in cookies  As cookies are generally sent
      in the clear, implementations must not MUST NOT store bearer tokens within
      them.

3.4.

   Issue short-lived bearer tokens  Using short-lived (one hour or less)
      bearer tokens can reduce the impact of one of them being leaked.
      The User-Agent flow should only issue short lived access tokens.

   Don't pass bearer tokens in page URLs  Browsers may not adequately
      secure URLs in the browser history.  If bearer tokens are passed
      in page URLs (typically as query string parameters), attackers
      might be able to steal them from the history data.  Instead, pass
      browser tokens in message bodies for which confidentiality
      measures are taken.

4.  IANA Considerations

   This document neither establishes new IANA registries nor adds new
   values to existing registries.

5.  References

5.1.  Normative References

   [I-D.ietf-httpbis-p1-messaging]
              Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
              Masinter, L., Leach, P., Berners-Lee, T., and J. Reschke,
              "HTTP/1.1, part 1: URIs, Connections, and Message
              Parsing", draft-ietf-httpbis-p1-messaging-09 (work in
              progress), March 2010.

   [OAuth2]   Hammer-Lahav, E., Ed., Recordon, D., and D. Hardt, "The
              OAuth 2.0 Protocol", 2010.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, June 1999.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC5849]  Hammer-Lahav, E., "The OAuth 1.0 Protocol", RFC 5849,
              April 2010.

   [W3C.REC-html401-19991224]
              Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01
              Specification", World Wide Web Consortium
              Recommendation REC-html401-19991224, December 1999,
              <http://www.w3.org/TR/1999/REC-html401-19991224>.

5.2.  Informative References

   [NIST800-63]
              Burr, W., Dodson, D., Perlner, R., Polk, T., Gupta, S.,
              and E. Nabbus, "NIST Special Publication 800-63-1,
              INFORMATION SECURITY", December 2008.

Appendix A.  Acknowledgements

   The following people contributed to preliminary versions of this
   document: Blaine Cook (BT), Brian Eaton (Google), Yaron Goland
   (Microsoft), Brent Goldman (Facebook), Raffi Krikorian (Twitter),
   Luke Shepard (Facebook), and Allen Tom (Yahoo!).  The content and
   concepts within are a product of the OAuth community, WRAP community,
   and the OAuth Working Group.

   The OAuth Working Group has dozens of very active contributors who
   proposed ideas and wording for this document, including: [[ If your
   name is missing or you think someone should be added here, please
   send Mike Jones a note - don't be shy shy! ]]

   Michael Adams, Andrew Arnott, Dirk Balfanz, Brian Campbell, Leah
   Culver, Bill de hOra, Brian Ellin, Igor Faynberg, George Fletcher,
   Tim Freeman, Evan Gilbert, Justin Hart, John Kemp, Eran Hammer-Lahav,
   Chasen Le Hara, Michael B. Jones, Torsten Lodderstedt, Eve Maler,
   James Manger, Laurence Miao, Chuck Mortimore, Justin Richer, Peter
   Saint-Andre, Nat Sakimura, Rob Sayre, Marius Scurtescu, Naitik Shah,
   Justin Smith, Jeremy Suriel, Christian Stuebner, Paul Tarjan, and
   Franklin Tse.

Appendix B.  Document History

   [[ to be removed by RFC editor before publication as an RFC ]]

   -01

   o  First public draft, which incorporates feedback received on -00
      including enhanced Security Considerations content.  This version
      is intended to accompany OAuth 2.0 draft 11.

   -00

   o  Initial draft based on preliminary version of OAuth 2.0 draft 11.

4.  Normative References

   [I-D.ietf-httpbis-p1-messaging]
              Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
              Masinter, L., Leach, P., Berners-Lee, T., and J. Reschke,
              "HTTP/1.1, part 1: URIs, Connections, and Message
              Parsing", draft-ietf-httpbis-p1-messaging-09 (work in
              progress), March 2010.

   [OAuth2]   Hammer-Lahav, E., Ed., Recordon, D., and D. Hardt, "The
              OAuth 2.0 Protocol", 2010.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, June 1999.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC5849]  Hammer-Lahav, E., "The OAuth 1.0 Protocol", RFC 5849,
              April 2010.

   [W3C.REC-html401-19991224]
              Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01
              Specification", World Wide Web Consortium
              Recommendation REC-html401-19991224, December 1999,
              <http://www.w3.org/TR/1999/REC-html401-19991224>.

Authors' Addresses

   Michael B. Jones (editor)
   Microsoft

   Email: mbj@microsoft.com
   URI:   http://self-issued.info/

   Eran Hammer-Lahav
   Yahoo!

   Email: eran@hueniverse.com
   URI:   http://hueniverse.com

   Dick Hardt
   independent

   Email: dick.hardt@gmail.com
   URI:   http://dickhardt.org/

   David Recordon
   Facebook

   Email: davidrecordon@facebook.com
   URI:   http://www.davidrecordon.com/